Remove `studio_post_duplicate` XBlock hook method

[kdmccormick]

This ad-hoc hook was added to account for edge-cases when duplicating or copy-pasting LibraryContentBlocks. It will no longer be necessary once we remove V1 content libraries.

[kdmccormick]

@Ian2012 LegacyLibraryContentBlock uses this hook, so we will have to keep it until we migrate V1 libs to V2 libs and migrate all LegacyLibraryContentBlock usages to ItemBankBlock usages.